explain xkcd - User contributions [en]

Talk:3097: Bridge Types

2025-06-03T08:07:04Z

172.71.183.12:

For budget overrun, see olympic stadium of Montreal, Quebec, Canada.

[[Special:Contributions/162.158.126.202|162.158.126.202]] 01:23, 3 June 2025 (UTC)

Very disappointed there's no bridge card game reference, but I guess that's not one of Randall's types of nerdiness :( [[Special:Contributions/172.71.254.203|172.71.254.203]] 01:45, 3 June 2025 (UTC)

I would like to note that cable stayed bridges, budget overrun here, are much cheaper than equivalent suspension bridges. It because they use less materials and can be built faster meaning less labor. [[Special:Contributions/172.69.58.51|172.69.58.51]] 01:50, 3 June 2025‎
:Tru dat in general, but I think that this is a reference to the {{w|Leonard_P._Zakim_Bunker_Hill_Memorial_Bridge|Zakim Bridge}} in downtown Boston, part of the {{w|Big_Dig|"Big Dig"}} project that became notorious for its budget overruns and related shenanigans. Given that Randall M. lives in Boston, that makes this panel something of an inside joke. [[Special:Contributions/172.71.147.224|172.71.147.224]] 03:15, 3 June 2025 (UTC)

The [[wikipedia:St. Louis Arch|St. Louis Arch]] is a repurposed-elevator-suspended-arch-but-without-the-base-and-wires bridge if you squint hard enough. The elevator is also fun. [[Special:Contributions/172.69.67.214|172.69.67.214]] 01:57, 3 June 2025 (UTC)

Nothing about a a [[wikipedia:Bridge circuit|bridge circuit]] or these [[Wikipedia:Bridges (disambiguation)|many]] [[wikipedia:other|other]] bridges either. Sigh. [[Special:Contributions/172.69.67.214|172.69.67.214]] 01:57, 3 June 2025 (UTC)
:And where, oh where, are Lloyd, Beau, Jeff, and Jordan? [[Special:Contributions/162.158.41.84|162.158.41.84]] 03:19, 3 June 2025 (UTC)

The L'Engle is a take off on a Wrinkle in time? But this one is in space? <small> -- [[User:162.158.91.124|162.158.91.124]] ([[User talk:162.158.91.124|talk]]) 02:26, 3 June 2025‎ <span style="color:grey; white-space:nowrap;">''(please sign your comments with <nowiki>~~</nowiki>~~)''</span></small>
:There's some space-warping in L'Engle's books. [[Special:Contributions/162.158.174.63|162.158.174.63]] 02:44, 3 June 2025 (UTC)

The "budget overrun" bridge doesn't really look like the Zakim bridge to me. It looks a lot like the Samuel Beckett Bridge in Dublin. I don't know what the budget of that bridge was, but according to wiki it cost 60 million euros, which sounds like a lot given that the bridge isn't all that long or wide. [[Special:Contributions/172.70.126.87|172.70.126.87]] 03:24, 3 June 2025 (UTC)
: Perhaps if Randall M. drew too close a likeness to the Zakim Bridge, he feared a visit from officials with lawyers and/or cement shoes. ("Only the paranoid survive ...") It seems, from a quick tour of the Internet, that words like "grandiose and overblown" are easily applied to cable-stayed bridge designs/aesthetics. I wasn't easily able to find information on budget overruns for these bridges, and see the commentator above who pointed out the lower costs overall of cable-stayed ''vs'' suspension bridges. But as a former resident of Greater Boston, I can report the pervasiveness of the Big Dig and its challenges, budgetary and otherwise, in local life and lore. [[Special:Contributions/172.68.22.108|172.68.22.108]] 04:32, 3 June 2025 (UTC)
::The cable-stayed bridge is the current darling of artists that accidentally went to engineering school, who are notorious for running over budget and behind schedule. [[User:RegularSizedGuy|RegularSizedGuy]] ([[User talk:RegularSizedGuy|talk]]) 04:40, 3 June 2025 (UTC)
: I can see the suggestion of the Beckett bridge, but in my eyes the obvious template would be [https://en.wikipedia.org/wiki/Erasmusbrug Rotterdam's Erasmus Bridge] [[User:Nachtvogel|Nachtvogel]] ([[User talk:Nachtvogel|talk]]) 06:00, 3 June 2025 (UTC)

I think the repurposed elevator should be considered a dig at Elon Musks The Boring Company, even though they tunnel rather then bridge
[[Special:Contributions/162.158.182.138|162.158.182.138]] 04:37, 3 June 2025 (UTC)

Added a bunch of explanations [[Special:Contributions/162.158.8.132|162.158.8.132]] 07:31, 3 June 2025 (UTC)

The Repurposed Elevator is actually a real thing! https://en.wikipedia.org/wiki/Schmid_Peoplemover It's not as strange as you think. It's a space effective, but too expensive solution to the problem of not making cramped railway crossroads more cramped. [[Special:Contributions/162.158.172.112|162.158.172.112]] 07:39, 3 June 2025 (UTC)
: I've added it to the list. Feel free to do such changes yourself if you know something that can contribute. --[[Special:Contributions/172.71.183.12|172.71.183.12]] 08:07, 3 June 2025 (UTC)

For the "Google Earth Bridge" remark, [https://www.fastcompany.com/90186315/the-strange-art-of-the-melting-bridges-of-google-earth this article] might work as a citation. [[User:Conster|Conster]] ([[User talk:Conster|talk]]) 07:57, 3 June 2025 (UTC)

Talk:3013: Kedging Cannon

2024-11-19T13:59:18Z

172.71.183.12:

First? [[User:PRR|PRR]] ([[User talk:PRR|talk]]) 02:05, 19 November 2024 (UTC)

Good essay on real-life (or this-world) kedging-- http://www.sailmagazine.com/cruising/cruising-tips/the-lost-art-of-kedging-how-to-set-a-kedge-anchor/ [[User:PRR|PRR]] ([[User talk:PRR|talk]]) 02:07, 19 November 2024 (UTC)

:Nice. I think I managed to somehow get in first (before I logged in); first time I've done so, so apologies for not knowing all the conventions. I think the title text is the main non-obvious thing, since the simile between a windmill's mechanical function and that of tacking seems clearly intentional,but I'm sure that could be edited to be clearer than my hasty writup. [[User:Mneme|Mneme]] ([[User talk:Mneme|talk]]) 02:10, 19 November 2024 (UTC)

For a brief, brief, moment, my brain failed to swap in and dredge up the memory of what kedging was, and I wondered if they were trying to use the aft cannon as a weak propulsion mechanism (hey, if it was a spacecraft…). And then I remembered what kedigng was and—DUH! [[User:JohnHawkinson|JohnHawkinson]] ([[User talk:JohnHawkinson|talk]]) 02:29, 19 November 2024 (UTC)

If it wasn't for kedging, I probably wouldn't be able to make it all the way through November. [[Special:Contributions/108.162.245.133|108.162.245.133]] 04:25, 19 November 2024 (UTC)
:Ha ha now exactly what you mean. The three longest month of the year are November November November... :-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 10:05, 19 November 2024 (UTC)
::Having a certain number of winches helps. [[Special:Contributions/172.68.23.92|172.68.23.92]] 10:44, 19 November 2024 (UTC)

Wow, whoever wrote the Speed and Economic Analysis section, you are amazing! [[Special:Contributions/172.71.98.135|172.71.98.135]] 05:13, 19 November 2024 (UTC)
:Sadly is was done by an anonymous IP address... But cool analysis. Have no idea what he actually calculates or if it is correct though ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 10:05, 19 November 2024 (UTC)
::The system described doesn't need two cannons, just two anchors and winches for continuous operation. As something of an anonymous IP address editor myself, I am checking the math and intend to parameterize the assumptions for different size boats, different headwinds, and other different parameters. If I am successful, I will log in to upload a graph showing when cannon kedging is superior (if it ever is....) [[Special:Contributions/172.68.23.92|172.68.23.92]] 10:44, 19 November 2024 (UTC)

I've got a strong hunch that this comic is about a captain who fails to take into account '''recoil / conservation of momentum''', which is a frequent mistake.
The cannon propels the anchor with great force, but, as it is connected to the ship, the ship is pushed back with the same force (minus some heat losses) ''before the anchor can settle''. Therefore, in this setup the ship will only move forward at all if the anchor ends up at a greater horizontal distance from the ship's original position than the distance between the ship's original and post-cannonshot positions.
The third panel, where the ship is drawn further to the left, indicates that the setback is significant and the ship only moves back and forth in the same place. Like, it ''literally'' takes forever. The title text is a hint: Only if the captain manages to harness a source of power that is independent from the vessel's movement (for example: wind from the wrong direction) this cannon will have any propagating effect - using this power for the winch, however, is just as futile as the kedging cannon itself.
For manual kedging, people heave the anchor to a boat and row out to a drop site, so it's not an issue.
Or am I missing something? [[User:Transgalactic|Transgalactic]] ([[User talk:Transgalactic|talk]]) 11:07, 19 November 2024 (UTC)
:No, this is incorrect; the center of mass of a kedging ship + anchor does not stay in the same place - the anchor has caught on some feature of the seabed so the CoM can move towards the anchor as it is winched in. Even a sea anchor would work so long as the drag in water is sufficiently greater than the drag in air, but at that point you're evolving towards re-inventing the oar.
::As the ship is much heavier than the anchor and it additionally experiences resistance from the water, the amount the ship moves back is much less than the amount the anchor moves forward. I don't know if the ship being drawn further to the left is supposed to indicate that it moved backwards, but even if you assume that the frames are supposed to represent the same area of the ocean (which would imply the ship has moved backwards) you can see that the anchor has landed to the right of the frame, so the ship will have moved forward at the end of the manoeuvre. [[Special:Contributions/172.68.205.92|172.68.205.92]] 12:03, 19 November 2024 (UTC)
:::I think the objection is based upon the "on a frictionless surface in a vacuum" thought experiment whereby you try to make progress from continually throwing a tethered object (with insufficient reach to get to the edge of said frictionless surface) then drawing it back in again. With absolutely nothing to assymeyrically provide grip, even throwing it out fast then drawing it back slowly won't get you either towards ''or away from'' the direction of throw after each complete cycle. (Whereas if you untethered it, or let the tether snap, you would at least get some residual frictionless+resistanceless Newtonian movement out of it and could coast back against the thrown direction until you sufficiently escaped the frictionless area.)
:::Of course, in the sea-going example there's various resisting effects that you can use to your advantage (assuming there aren't active wind/water effects sufficient to overcome your desired vector of gains). Either differential between air and water (sea-anchor flies out with less resistance than it encounters when you drag it back in, or just set up something to row/paddle-wheel the same sort of effect of uneven push/pull reciprocation), an assymetrically varying object of resistence (opening/closing umbrella, parachute envelope or, potentially again, sea-anchor equivalent), an off-direction effect that creates a perpendicular force you require "waggly-tail" oaring, or how a boat stays upright so that a relatively turning screw creates propulsive thrust) or even use time-/force-dependent viscosity (shoving hard in one direction creates a different total resistance from pulling the exact same (directionally agnostic) mass softly back again in the opposite direction; rinse and repeat to accumulate whatever differential effect that you generate). Several other options suggest themselves, with fully closed-cycle reciprocations.
:::Though what we have here is a partial 'open-cycle' system. The expelled 'exhaust' of the fired cannon actually acting ''against'' the throw-and-drag process, but probably only marginally, and ''could'' be factored in as a beneficial force either by using sufficient waste-gas "blow back" diverters to extract cannon-direction momentum (not just recoilless, but 'anti-directional recoil', like jet engine reverse-thrust 'scoops' do to aid deceleration once safely touched down upon landing) in just the right way or where the "firing is through a hard medium, but the pull-back is through an easy one" (e.g. direct your projectile directly out-but-down into the water, but have it buoyant enough to breach the surface (or even rise, Zeppelin-like, completely out into the air!) so you can retrieve it with far less dragging effort - then you're travelling in the opposite direction from what you're firing, adding 'redeployable thrust' to the expended 'rocket reaction thrust').
:::Anyway, all that aside, I think that's where Transgalactic was headed (their original "am I missing something?" bit, before all my "no, except..." caveats as to various possible missing bits). Hopefully helps a bit. Even if none of it ''technically'' helps in a totally reactionless (non-depleting) space-thrusting scenario, which of course would be a useful thing to have. (We probably need a way to decouple gravitational mass from inertial mass, and selectively so, to do ''that!'') [[Special:Contributions/172.70.46.222|172.70.46.222]] 13:43, 19 November 2024 (UTC)

Maybe instead of using a cannon, we could use a ballista? Saves gunpowder, but requires human labor. I think that would still be more efficient. --[[User:Coconut Galaxy|Coconut Galaxy]] ([[User talk:Coconut Galaxy|talk]]) 11:44, 19 November 2024 (UTC)

It would be more efficient to have the sails furled. Every time the anchor is lifted the wind will push the boat back again. {{unsigned ip|172.69.214.135|12:27, 19 November 2024}}